This disclosure generally relates to a magnetorheological fluid damper and more particularly, to a magnetorheological fluid damper providing an increased shear interface area per unit volume and/or mass of the damping device.
Magnetorheological (MR) fluids belong to a class of controllable fluids. The essential characteristic of these fluids is their ability to reversibly change from a free-flowing, linear, viscous liquid to a semi-solid with a controllable yield strength in milliseconds when exposed to a magnetic field. In the absence of an applied field, MR fluids are reasonably well approximated as Newtonian liquids.
A typical MR fluid has about 20 to about 40 percent by volume of relatively pure, soft iron particles, typically about 3 to about 5 microns, suspended in a carrier liquid such as mineral oil, synthetic oil, water, or glycol. A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear. The ultimate strength of the MR fluid depends on the square of the saturation magnetization of the suspended particles.
MR fluids made from iron particles typically exhibit maximum yield strengths of 30–90 kPa for applied magnetic fields of 150–250 kA/m (1 Oe·80 A/m). MR fluids are not highly sensitive to moisture or other contaminants that might be encountered during manufacture and use. Furthermore, because the magnetic polarization mechanism is not affected by the surface chemistry of surfactants and additives, it is a relatively straightforward matter to stabilize MR fluids against particle-liquid separation in spite of the large density mismatch.
Most devices employ MR fluids in a valve mode, direct-shear mode, or combination of these two modes. Examples of valve mode devices include servovalves, dampers, and shock absorbers. Examples of direct-shear mode devices include clutches, brakes, and variable friction dampers. The maximum stroking force that an MR damper can provide generally depends on the MR fluid properties, the flow pattern, and the size of the damper.
However, the range of stroking forces achievable with current MR materials, flow patterns, and damper geometries are not sufficient for these devices to be practical for some applications such as, for example, crash management applications. For some applications, an increased shear interface per unit volume is desirable since it directly increases the available stroking force.